1. Field of the Invention
The present invention relates to a frequency conversion apparatus for converting the frequency of an outer input signal in which element signals having a predetermined frequency bandwidth are sequentially arranged in a predetermined frequency interval.
2. Description of the Related Art
Regarding a system for transmitting and processing signals including frequency components, a signal having a relatively high frequency is traditionally used in a wireless system, whereas a signal having a lower frequency is used in a wire system. Accordingly, as described in, for example, Japanese Unexamined Patent Application Publication No. 2000-299848 (Publication 1), a communication system in which a wireless communication is carried out in an access port to a terminal station of the respective homes or the like via a bi-directional CATV cable network, that is, a system including both a wireless system and a wire system requires the frequency conversion for the signals.
In the case of an outer input signal including a plurality of signals having a different frequency from each other, only a signal having a desired frequency is conventionally selected (extracted) from the input signal, and then generally processed after the frequency conversion.
FIG. 5 shows a block diagram of a heterodyne type frequency selection means, which is widely used in the conventional system. As shown in FIG. 5, the heterodyne type frequency selection means comprises an oscillator 52 for supplying a signal having a predetermined resonant frequency, a frequency mixer 51 and a filter 53, wherein a desired element signal can be selected (extracted) from an input signal in which signals (hereinafter being referred to as element signals) having a predetermined frequency bandwidth (which is not always fixed) are sequentially arranged in a predetermined frequency interval (the interval not always being fixed). In other words, an outer input signal (having a frequency fin) and an output signal (having a frequency fL) from the oscillator 52 are both input into said frequency mixer 51, and then the outer input signal is converted to a signal including two signals which have a frequency difference (|fin−fL|) and a frequency sum of (fin+fL) of the outer input signal and the output signal, respectively. Thereafter, the desired element signal is selected (extracted) from the signal thus converted by the filter 13 through which only the desired element signal is capable of passing. In this case, the frequency of the oscillator 52 is preset such that the desired element signal resides in the pass frequency bandwidth of the filter 13.
In Japanese Unexamined Patent Application Publication No. 9-93152 (Publication 2), a double conversion television tuner, in which a frequency conversion is made by means of the heterodyne type frequency selection means, has been disclosed. In this tuner, a desired channel signal is selected from a broadcasting signal including a plurality of channel signals (corresponding to said element signals), thereby allowing the frequency conversion to be made. A frequency conversion apparatus can be constituted by providing several double conversion television tuners (for example, the number of the tuners being the same as that of channels) to select channel signals different from each other, wherein signals having a higher frequency are initially converted to the signals having a lower frequency which can be processed with ease in a wire system, and then the signals thus converted are again converted into signals having an other higher frequency.
Since, however, there is a restriction for the frequency bandwidth used in the communication, it is desirable that the output signals after the frequency conversion have the element signals in a finer density (the frequency interval of the element frequencies being small or adjacent to each other) to reduce the total bandwidth of the frequency used. Any means for attaining such a requirement cannot be found in Publication 2.